It is known from GB-A-2262861 to suggest a panel-form loudspeaker comprising:
a resonant multi-mode radiator element being a unitary sandwich panel formed of two skins of material with a spacing core of transverse cellular construction, wherein the panel is such as to have ratio of bending stiffness (B), in all orientations, to the cube power of panel mass per unit surface area (.mu.) of at least 10; PA1 a mounting means which supports the panel or attaches to it a supporting body, in a free undamped manner; PA1 and a electro-mechanical drive means coupled to the panel which serves to excite a multi-modal resonance in the radiator panel in response to an electrical input within a working frequency band for the loudspeaker.
U.S. Pat. No. 4,506,117 of MULTIPHONIE discloses an electroacoustic transducer comprising an inertial mass adapted to be attached rigidly by its base plate to a panel to be vibrated.